I mostly agree with everything you said with only a couple of amendments.
1] Belinda is a Top Tier Event Unit once you awaken her and get her +7% Witch Damage passive. Although you actually don't need to place her on the map to benefit from it just include her in your squad she is still a Platinum Witch and better basic stats that most alternatives.

2] I actually already have Marnie but still find myself using Waltz more because while Marnie may have slightly better total stats my Waltz is max cost down and about 5/10 in skill. Which including her longer duration effect makes her more useful to me in most situations... I will probably go back to favoring Marnie once I awaken her and get her passive -1 cost to melee units.

3] You missed Out: Echidna (Heal/Mage), Odette (Mage), Maribel (Burst Tank), Len (Rear Strategist)
E = Has top tier Heal Potential with only a slightly reduced Range with ability to burst out some AoE Magic Damage.
O = Is a Platinum Level Mage with a Team Effecting Magic Resistance Buff.
M = While Poorly stated for a Tank normally gains massive stat boosts under effect.
L = I've only recently started to appreciate Rear Strategists I initially avoided her because her attack power was low. I wasn't aware then that she was a Double Shooting unit... I've also gotten more used to using skills and the class's effect to reduce the cooldown time is helpful when your trying to use skills frequently.


From earliest to latest, these are the useful event units, IMO.
[I rejigged the list with Class info and my preference order. However Claudia probably should be Higher but I have Awakened Sybilla.]

Aria (Soldier)
Immortal Princess Karma (Princess Ganker)
Anya (Princess Tank)
Solano (Summoner)
Rika (Automata)
Claudia (Princess)
Imelia (Bandit)
Shiho (Witch/Healer)

These units are also useful if you don't have plat counterparts or a princess.

Belinda (Witch)
Waltz (Dancer)
Shuka (Advance Stratagist)
Horace (Magic Fencer)
Rowanna (Avenger)